This invention relates, in general, to the oxidative dehydrogenation of hydrocarbons. More particularly, the present invention relates to rare earth catalysts that provide unusually high selectivity to higher hydrocarbons and/or lower olefins when used for the oxidative dehydrogenation of a lower hydrocarbon at elevated pressure. Accordingly, the rare earth catalysts of the invention are particularly useful for coupling methane by oxidative dehydration to form ethane, ethylene and higher hydrocarbons, and for the oxidative dehydrogenation of ethane to form ethylene.
Methane is an attractive raw material because it is widely available and inexpensive, but it is used mainly as a fuel. Natural gas liquids (ethane, propane, butane and higher hydrocarbons) are the major raw material for ethylene and propylene, from which many petrochemicals are produced. But the supply of natural gas liquids has not kept pace with increasing demand for olefins, so more costly cracking processes that use naphtha from petroleum are being commercialized. Therefore, the development of economical processes for manufacturing olefins and other hydrocarbons from methane is highly desirable.
Methane has low chemical reactivity, so severe conditions are required to convert it to higher hydrocarbons. Oxidative dehydrogenation is favored because conversion is not thermodynamically limited and reactions are exothermic. But selectively producing ethylene, ethane, and higher hydrocarbons by partial oxidation while avoiding complete oxidation to carbon oxides is difficult to achieve. Accordingly, those skilled in the art have expended much effort in attempts to develop selective catalysts for methane coupling. Rare earth oxycarbonate and oxide catalysts have been of particular interest.
U.S. Pat. No. 4,929,787 discloses a catalyst for oxidative coupling that contains at least one rare earth metal carbonate, which is defined to include simple carbonates and oxycarbonates and which comply approximately with the stoichiometric formulas M2(CO3)3, M2O2CO3, M2O(CO3)2, or M(OH)(CO3), which may be characterized by elementary analysis, where M is at least one rare earth metal. The rare earth oxycarbonates, M2O2CO3, are preferred, with lanthanum oxycarbonate, La2O2CO3, being most preferred. Only lanthanum, neodymium, and samarium are used in the examples. The catalysts may be prepared in several ways by thermal decomposition of a rare earth metal compound: carbonates may be directly decomposed; hydroxides, nitrates, carbonates, or carboxylates may be added to a solution of polycarboxylic acid (citric), dried, and roasted under vacuum or in air; carbonates, hydroxides, or oxides may be added to an acid (acetic), dried, and decomposed in air; carbonates or carboxylates (acetates) may be dissolved into aqueous carboxylic acid (formic or acetic), impregnated onto a carrier, and heated in air; or oxides may be contacted with carbon dioxide. These methods all specify decomposing the precursors at a temperature of 300xc2x0 to 700xc2x0 C., but the examples all use 525xc2x0 to 600xc2x0 C. The decomposition may be done outside or inside the reactor before passing the reacting gas mixture over the catalyst. In one example, the La2O2CO3 catalyst was prepared by heating at 120xc2x0 C. an acetic acid solution containing lanthanum acetate, reducing the volume of the solution by aspiration, drying the material at 150xc2x0 C. under high vacuum, crushing the resultant foam to fine powder, and roasting the powder in air at 600xc2x0 C. for two hours. In another example, the reactor was charged with anhydrous lanthanum acetate and treated with helium at 525xc2x0 C. for one hour to form the La2O2CO3 catalyst. The catalyst may also contain one or more alkaline earth metal (Be, Mg, Ca, Sr, Ba) compounds to improve selectivity and a Group IVA metal (Ti, Zr, Hf) to increase activity. The reaction temperature specified is 300xc2x0 to 950xc2x0 C., preferably 550xc2x0 to 900xc2x0 C.; the examples are mainly at 600xc2x0 to 750xc2x0 C., but the catalysts are selective at temperatures exceeding 900xc2x0 C. as well. The reaction pressure specified is 1 to 100 bars, particularly 1 to 20 bars, but the examples are all at atmospheric pressure. Carbon dioxide may be beneficially added (up to 20%) to the reaction gases as a diluent to increase yield by moderating the bed temperature and as a constituent to maintain a high activity of the carbonate catalyst. These catalysts are utilized in the related processes disclosed in U.S. Pat. Nos. 5,025,108 and 5,113,032.
The effect of reaction pressure on a catalyst disclosed in U.S. Pat. No. 4,929,787 was studied in M. Pinabiau-Carlier, et al., xe2x80x9cThe Effect of Total Pressure on the Oxidative Coupling of Methane Reaction Under Cofeed Conditionsxe2x80x9d, in A. Holmen, et al., Studies in Surface Science and Catalysis, 61, Natural Gas Conversion, Elsevier Science Publishers (1991). The catalyst (A) was a mechanical mixture of lanthanum oxycarbonate and strontium carbonate that was calcined in air at 600xc2x0 C. for two hours. Increasing the pressure substantially decreased the selectivity to C2+ hydrocarbons (reaction temperature of 860xc2x0 C. from 72% at 1 bar to 39% (constant flow rate) or 35% (increased flow rate for constant conversion) at 7.5 bar (94 psig). Another catalyst (B) was a magnesia support impregnated with aqueous lanthanum and strontium nitrates and then calcined at 800xc2x0 C. for two hours. This calcination temperature is above the maximum specified calcination temperature of 700xc2x0 C. disclosed in U.S. Pat. No. 4,929,787 for producing oxycarbonate, and is a temperature at which predominantly lanthanum oxide, La2O3, is expected to form. The preparation furthermore did not include a carbon source from which oxycarbonate could be formed from the nitrate. Increasing the pressure significantly decreased the C2+ selectivity (900xc2x0 C. from 79% at 1.3 bar to 65% at 6 bar (72 psig) with constant flow rate. The study concluded that the reaction should be operated at pressures below 3 bar (29 psig).
A catalyst disclosed in U.S. Pat. No. 4,929,787 was used to study the effect of adding 10% ethane to oxidative coupling and pyrolysis reactors in series in H. Mimoun, et al., xe2x80x9cOxidative Coupling of Methane Followed by Ethane Pyrolysisxe2x80x9d, Chemistry Letters 1989: 2185. The catalyst was a mechanical mixture of lanthanum oxycarbonate and strontium carbonate. Ethane added to the coupling reactor (880xc2x0 C. and one atmosphere) decreased methane conversion and increased ethylene and carbon monoxide production. The study concluded that oxygen preferentially dehydrogenates ethane instead of coupling methane; ethane is best separated from the natural gas feed and supplied to just the pyrolysis reactor, where it is cracked with high selectivity to olefins, as disclosed in U.S. Pat. No. 5,025,108.
U.S. Pat. No. 5,061,670 discloses a method for preparing a cocatalyst of lanthanide and alkaline-earth metal carbonates and/or oxycarbonates, which comprises forming an aqueous solution of lanthanide and alkaline-earth metal chlorides; adding alkali metal carbonate and optionally hydroxide to coprecipitate carbonates and/or hydroxycarbonates at a basic pH above 8; separating the coprecipitate from the reaction medium; washing away the alkali metal chlorides formed; and drying and calcining the coprecipitate at 400xc2x0 to 1000xc2x0 C. in air or an inert atmosphere. Scandium, yttrium, and lithium may be added as promoters. The examples form cocatalysts of barium with lanthanum or samarium.
Cocatalysts of BaCO3 and La2O2CO3 were studied in U. Olsbye, et al., xe2x80x9cA Comparative Study of Coprecipitated BaCO3/La2On(CO3)m Catalysts for the Oxidative Coupling of Methanexe2x80x9d, Catalysis Today 13: 603 (1992). They were prepared by mixing aqueous BaCl2 and LaCl3 with NaOH and Na2CO3 at a pH above 8, washing and drying the precipitate, and calcining it at 500xc2x0 C. in air. The reaction was done at 750xc2x0 to 850xc2x0 C. at atmospheric pressure. The catalysts were small crystals (300-500 xc3x85) of BaCO3 and La2O2CO3 (various polymorphs) and some La2O3 after calcination, and were BaCO3 and La2O3 after reaction. The tendency of La2O2CO3 to convert to La2O3 was confirmed by thermogravimetric analyses. Surface areas were  less than 16 m2/g after calcination. The areas decreased during reaction as crystal size grew.
Rare earth oxides have been used as catalysts for methane coupling at atmospheric pressure in many studies. They have been prepared from a variety of rare earth compounds, such as carbonates, hydroxides, nitrates, acetates, and oxalates, by calcination at high temperature in air or another atmosphere, such as nitrogen or helium. The phase composition of these catalysts is known to be highly dependent on the preparation method. Lanthanum oxide in particular is sensitive to exposure to atmospheric water vapor and carbon dioxide, which can convert the oxide over time to a partially carbonated hydroxide. Hydration and carbonation can also occur during catalysis. Commercially prepared oxides are often recalcined as received or after hydrothermal treatment before they are used as catalysts. The surface area of the prepared catalyst generally ranges from 3 to 10 m2/g, with some higher or lower values reported. Surface area decreases with higher calcination temperature and during reaction. The rare earth oxides have been promoted mainly by alkali metal (Li, Na, K, Rb, Cs) and alkaline earth metal (Be, Mg, Ca, Sr, Ba) compounds, mostly in the form of oxides or carbonates. Other promoter compounds have contained elements of Group IIIA (Sc, Y), Group IVA (Ti, Zr, Hf), manganese, Group IB (Cu, Ag, Au), Group IIB (Zn, Cd, Hg), Group IIIB (Al, In), Group IVB (Si, Ge, Sn, Pb), and Group VB (P, Sb, Bi).
U.S. Pat. Nos. 4,499,323; 4,499,324; 4,727,211; 4,727,212; 5,146,027; 5,210,357; 5,567,667; and 5,712,217 disclose the rare earth oxides of lanthanum, cerium, praseodymium, and terbium as catalysts for methane coupling. However, several literature studies report that deleterious effects result from the use of rare earth oxides for the oxidative coupling reaction of methane under elevated pressure.
A. Ekstrom, et al., xe2x80x9cEffect of Pressure on the Oxidative Coupling Reaction of Methanexe2x80x9d, Applied Catalysis 62: 253 (1990), studied the effect of pressure on oxidative coupling by Sm2O3 and SrCO3/Sm2O3. Increasing the pressure to 87 psi significantly increased the importance of the uncatalyzed combustion reaction. This could be reduced by using high linear velocities, but increasing the pressure under these conditions still depressed the C2+ selectivity and the catalyst activity.
D. E. Walsh, et al., xe2x80x9cDirect Oxidative Methane Conversion at Elevated Pressure and Moderate Temperaturesxe2x80x9d, Industrial and Engineering Chemistry Research 31: 1259 (1992), studied the effect of high pressure on oxidative coupling by Sm2O3. The C2+ selectivity declined from 55-60% at atmospheric pressure (800-850xc2x0 C. to 36% at 900 psi (550xc2x0 C.). However, at 900 psi, the non-catalyzed reaction gave 32% selectivity, with the gain being in ethane rather than ethylene. Therefore at high pressure the catalyst had little effect on the coupling reaction. Similarly, D. E. Walsh, et al., xe2x80x9cPressure, Temperature, and Product Yield Relationships in Direct Oxidative Methane Conversion at Elevated Pressures and Moderate Temperaturesxe2x80x9d, Industrial and Engineering Chemistry Research 31: 2422 (1992), obtained only 13% C2+ selectivity for oxidative coupling at 450 psi (630xc2x0 C. by using Sm2O3, with little ethylene produced (2.5%).
Clearly, there is a need for improved catalysts for the oxidative dehydrogenation of hydrocarbons and, in particular, for producing ethylene, ethane, and higher hydrocarbons from methane by oxidative dehydrogenation coupling. Such catalysts would provide high selectivity for oxidative dehydrogenation reactions and would enable these reactions to be carried out at elevated pressure instead of at atmospheric pressure. Improved catalysts would also have high activity at low temperature, operate at economical conversion levels, and remain stable during long-term operation. These catalysts must also be suitable for large-scale commercial production.
The present invention meets the above-noted objects by providing, in one aspect, catalysts which are highly selective for the oxidative dehydrogenation of lower hydrocarbons to produce higher hydrocarbons and/or lower olefins. The invention further provides methods for preparing such catalysts and processes for using the catalyst in the oxidative dehydrogenation of lower hydrocarbons. As used herein, the term xe2x80x9clower hydrocarbonxe2x80x9d includes lower alkanes (typically C1-C4 alkanes), alkyl aromatics (typically aromatics having C1-C4 alkyl appendages), and cyclic compounds. The term xe2x80x9chigher hydrocarbonxe2x80x9d means a hydrocarbon having a greater number of carbon atoms than the lower hydrocarbon which undergoes oxidative dehydrogenation (e.g., the coupling of methane to form ethane, ethylene and other higher hydrocarbons). The term xe2x80x9clower olefinxe2x80x9d refers to an olefin having the same number of carbon atoms as the lower hydrocarbon which undergoes oxidative dehydrogenation (e.g., the oxidative dehydrogenation of ethane to form ethylene).
In one embodiment, the catalyst taught by the invention comprises a nonstoichiometric rare earth oxycarbonate of the formula MXCYOZ having a disordered and/or defect structure, wherein M is at least one rare earth element selected from the group consisting of La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm; X=2, Z=3+AY; A is less than about 1.8, and Y is the number of carbon atoms in the oxycarbonate. When used for the oxidative dehydrogenation of a lower hydrocarbon at a pressure above about 100 psig, the catalyst has a selectivity of at least about 40% to at least one higher hydrocarbon and/or lower olefin. The catalyst may further comprise a cocatalyst containing at least one metal selected from the group consisting of V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Co, Ni, Cu, Zn, Sn, Pb, Sb, and Bi. The cocatalyst may also include at least one alkali metal or alkaline earth metal.
In another embodiment, a catalyst according to the invention comprises an oxycarbonate, hydroxycarbonate, and/or carbonate of at least one rare earth element selected from the group consisting of La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm. When used for the oxidative dehydrogenation of a lower hydrocarbon, the catalyst exhibits higher selectivity to at least one higher hydrocarbon and/or lower olefin at a pressure above about 100 psig than the catalyst or a precursor of the catalyst exhibits at a pressure in the range of about atmospheric pressure to about 25 psig. When operating at a pressure above about 100 psig, the catalyst has a selectivity of at least about 40%.
In still another embodiment, the catalyst taught by the invention comprises: (1) an oxycarbonate, hydroxycarbonate and/or carbonate of at least one rare earth element selected from the group consisting of La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm; and (2) a cocatalyst including at least one metal selected from the group consisting of V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Co, Ni, Cu, Zn, Sn, Pb, Sb, and Bi. When used for the oxidative dehydrogenation of a lower hydrocarbon the catalyst has a selectivity of at least about 40% to at least one higher hydrocarbon and/or lower olefin.
In yet another embodiment, the catalyst of the invention comprises: (1) an oxide of at least one rare earth element selected from the group consisting of La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm; and (2) a cocatalyst including at least one metal selected from the group consisting of V, Nb, Ta, Cr, Mo, W, Re, Fe, Co, and Ni. The catalyst, when used for the oxidative dehydrogenation of said lower hydrocarbon, has a selectivity of at least about 40% to at least one higher hydrocarbon and/or lower olefin.
As previously noted, the invention is also directed to methods for preparing catalysts selective for the oxidative dehydrogenation of lower hydrocarbons and to processes for using these catalysts. These methods and processes will be disclosed in detail below in connection with the detailed discussion of the various embodiments of the catalysts taught by the invention.